Implantable cardiac systems, such as implantable defibrillators, are designed to identify and treat certain cardiac arrhythmias. Arrhythmias are generally identified by the analysis of electrical signals in the patient, though other signals (movement, sounds, etc.) may also be integrated into the analysis. The electrical signals which emanate from the heart during both non-treatable and treatable cardiac activity can vary widely. New and alternative methods for identifying treatable cardiac activity are needed.